The use of cathodoluminescent materials (phosphors), substances that transform energy into light, has become ubiquitous in consumer electronic displays, including the wrist watch, electronic equipment status displays, and lap-top computers. Phosphor films are also important in defense applications, ranging from high definition table top map displays to helmet-mounted and "headsup" displays.
One of the most common uses for phosphor films is in computer screens. A major limitation in the use of phosphor films is that their luminescence degrades over time. The mechanism(s) for this degradation is poorly understood, but the degree of degradation is related to the "Coulombic aging" of the phosphor. "Coulombic aging" is the current multiplied by time and expressed as Coulombs of charge per unit area.
The most widely used phosphors in the manufacture of computer screens are zinc sulfides because their quantum efficiency is known to be the highest, about 22%, compared to some other oxysulfides or silicate phosphors which have a quantum efficiency of about 2-12%. A major problem with the zinc sulfide phosphors, however, is their Coulombic aging which leads to a loss of efficiency and brightness with use. Zinc sulfide phosphors lose about half of their brightness, or 50% of their initial efficiency, after only 30-50 Coulombs/cm.sup.2 charge loading. Since 1 Coulomb/cm.sup.2 corresponds to about 250 hours of cathode ray tube (CRT) phosphor screen use, the life expectancy of a zinc sulfide phosphor screen typically does not exceed 12,500 hours of operational life. Zinc sulfides, being the standard cathodoluminescent material used in phosphor screens, have dictated the commercial requirement that CRTs have an operational life span of 10,000 hours.
The Coulombic aging of phosphors has become increasingly critical for the computer industry with the development of the Flat Panel Display (FPD), which operates at higher current densities than CRTs and therefore has an even shorter life span than CRTs. For example, the operational voltage for FPDs range from 100 V to 1,000 V and require higher current densities to achieve the same power loading. Under these operational conditions, the phosphors (especially the more efficient sulfide and oxysulfide phosphors) quickly degrade due to Coulombic aging and saturation. Thus, extending the life span of the FPDs, which could lead to the commercial realization of full-color FPDs, would require the development of phosphors that are not as susceptible to Coulombic aging or could operate at lower current densities.
To date, the operational voltage of FPDs has been limited by the physical and chemical characteristics of the phosphors used. Commercially available phosphors have a high threshold voltage typically ranging from about 100 to 120 electron volts (eV). Since currently available CRTs and FPDs use phosphors having a high threshold voltage, the operation of those display devices is highly power consumptive.
Thus, there is a pressing need for phosphor films that exhibit decreased Coulombic aging and/or lower threshold voltages.